Our research in the area of viral nucleocapsid assembly has focused on two systems: (i) bacteriophage T4 capsid assembly, and (ii) murine leukemia virus morphogenesis. For the T4 system, we have utilized mutants blocked in the packaging of DNA into a capsid precursor. Such intermediate structures were isolated and utilized as substrates for probing the physicochemical nature of the initial association of a T4 DNA fragment(s) (about 12-32% of a phage equivalent) with proteins inside the capsid. By further isolating and characterizing this internal nucleoprotein complex, we are continuing to analyze the nature of nucleic acid-protein interactions required for initiation of DNA packaging in T4. For murine leukemia virus assembly we are continuing to conduct a detailed electron microscope study of a Moloney-producing JLSV9 cell line that can be synchronized by serum starvation. After synchrony there appear to be two modes of virus production: one predominantly from the outer perimeter of the cell, and a second which involves transport of the virus via phagosomes to the surface. The initial mode leads to an accumulation of immature or "A"-type particles and is resistant to Actinomycin-D treatment in contrast to the second mode. Both peaks are sensitive to cycloheximide treatment. We are now examining continuous cultures of these infected cells to determine whether there is any relation to either of these modes and the cell cycle. BIBLIOGRAPHIC REFERENCES: McMillan, P.N., and R.B. Luftig (1975). Preservation of membrane ultrastructure with aldehyde or imidate fixatives. J. Ultrastructure Res., 52: 243-260. Luftig, R.B., and R. R. Weihing (1975). Adenovirus binds to rat brain microtubules in vitro. J. Virology, 16: 696-706.